Didier Paillard

The timing of the last deglaciation in North Atlantic climate records

To determine the mechanisms governing the last deglaciation and the sequence of events that lead to deglaciation, it is important to obtain a temporal framework that applies to both continental and marine climate records. Radiocarbon dating has been widely used to derive calendar dates for marine sediments, but it rests on the assumption that the ‘apparent age’ of surface water (the age of surface water relative to the atmosphere) has remained constant over time. Here we present new evidence for variation in the apparent age of surface water (or reservoir age) in the North Atlantic ocean north of 40° N over the past 20,000 years. In two cores we found apparent surface-water ages to be larger than those of today by 1,230 ± 600 and 1,940 ± 750 years at the end of the Heinrich 1 surge event (15,000 years BP) and by 820 ± 430 to 1,010 ± 340 years at the end of the Younger Dryas cold episode. During the warm Bølling–Allerød period, between these two periods of large reservoir ages, apparent surface-water ages were comparable to present values. Our results allow us to reconcile the chronologies from ice cores and the North Atlantic marine records over the entire deglaciation period. Moreover, the data imply that marine carbon dates from the North Atlantic north of 40° N will need to be corrected for these highly variable effects.

Synchronous Change of Atmospheric CO2 and Antarctic Temperature During the Last Deglacial Warming

No Leader to Follow Changes in the concentration of atmospheric CO2 and surface air temperature are closely related. However, temperature can influence atmospheric CO2 as well as be influenced by it. Studies of polar ice cores have concluded that temperature increases during periods of rapid warming have preceded increases in CO2 by hundreds of years. Parrenin et al. (p. 1060; see the Perspective by Brook) present a revised age scale for the atmospheric component of Antarctic ice cores, based on the isotopic composition of the N2 that they contain, and suggest that temperature and CO2 changed synchronously over four intervals of rapid warming during the last deglaciation. Rising air temperature did not lead the increase of atmospheric carbon dioxide concentration during the last deglaciation. [Also see Perspective by Brook] Understanding the role of atmospheric CO2 during past climate changes requires clear knowledge of how it varies in time relative to temperature. Antarctic ice cores preserve highly resolved records of atmospheric CO2 and Antarctic temperature for the past 800,000 years. Here we propose a revised relative age scale for the concentration of atmospheric CO2 and Antarctic temperature for the last deglacial warming, using data from five Antarctic ice cores. We infer the phasing between CO2 concentration and Antarctic temperature at four times when their trends change abruptly. We find no significant asynchrony between them, indicating that Antarctic temperature did not begin to rise hundreds of years before the concentration of atmospheric CO2, as has been suggested by earlier studies.

Changes in Meridional Temperature and Salinity Gradients in the North Atlantic Ocean (30°–72°N) during the Last Interglacial Period

Eight deep-sea sediment cores from the North Atlantic Ocean ranging from 31° to 72°N are studied to reconstruct the meridional gradients in surface hydrographic conditions during the interval of minimum ice volume within the last interglacial period. Using benthic foraminiferal δ18O measurements and estimates of Sea Surface Temperature (SST) and Sea Surface Salinity (SSS), we show that summer SSTs and SSSs decreased gradually during the interval of minimum ice volume at high-latitude sites (52°–72°N) whereas they were stable or increased during the same time period at low-latitude sites (31°–41°N). This increase in meridional gradients of SSTs and SSSs may have been due to changes in the latitudinal distribution of summer and annual-average insolation and associated oceanic and atmospheric feedbacks. These trends documented for the Eemian ice volume minimum period are similar to corresponding changes observed during the Holocene and may have had a similar origin.

Constraints on the duration and freshwater release of Heinrich event 4 through isotope modelling

Heinrich events—abrupt climate cooling events due to ice-sheet instability that occurred during the last glacial period—are recorded in sediment cores throughout the North Atlantic Ocean. Modelling studies have described likely physical mechanisms for these events, but the quantitative characteristics of Heinrich events are less well known. Here we use a climate model of intermediate complexity that explicitly calculates the distribution of oxygen isotopes in the oceans to simulate Heinrich event 4 at about 40,000 yr ago. We compare an ensemble of scenarios for this Heinrich event with oxygen isotope data measured in foraminiferal calcite of a comprehensive set of sediment cores. From this comparison, we obtain a duration of 250 ± 150 yr and an ice release of 2 ± 1 m sea-level equivalent for Heinrich event 4, significantly reducing the uncertainties in both values compared to earlier estimates of up to 2,000 yr and 15 m of sea-level equivalent ice release, respectively. Our results indicate that the consequences of Heinrich events may have been less severe than previously assumed, at least with respect to Greenland climate and sea level.

Does mean annual insolation have the potential to change the climate

Long-term climatic changes, such as glacial-interglacial cycles, are usually explained in term of changes in solar energy received at the top of the atmosphere. In particular, daily insolation in the high Northern Hemisphere latitudes during summer is widely used in interpreting palaeoclimate records. This insolation forcing is strongly marked by changes in precession. However, some climate variations are much more imprinted by changes in obliquity. This was the case for sea surface temperature in the North Atlantic during the Eemian period, as well as for the Vostok ice core deuterium excess history over the last 250 ka. Therefore, we investigate the insolation forcing in order to identify characteristics that could explain the observed climate response. This is mainly the case for annual mean insolation variations. Simple hypotheses for how this forcing could act on climate are also suggested, these being mainly related to changes in the moisture transport induced by the annual insolation gradient between low and high latitudes. Along these lines, a simple conceptual model of annual mean temperature variations illustrates the role of annual mean insolation on climate

Amplitude and phase of glacial cycles from a conceptual model

Abstract The astronomical theory of climate, in which the orbital variations of the Earth are taken to drive the climate changes, explains many features of the paleoclimatic records. Nevertheless, the precise link between insolation variations and climatic changes during the Quaternary remains mysterious in several aspects. In particular, the largest sea level changes of the past million years occurred when insolation variations were minimal, like during stage 11, and vice versa like during stage 7. Moreover, recent data from terminations II and III show surprising phase lead and lag between insolation and sea level variations. To explain these paradoxical amplitude and phase modulations, we suggest here that deglaciations started when a combination of insolation and ice volume was large enough. To illustrate this new idea, we present a simple conceptual model that simulates the sea level curve of the past million years with very realistic amplitude modulations, and with good phase modulations.

Progress in Paleoclimate Modeling

This paper briefly surveys areas of paleoclimate modeling notable for recent progress. New ideas, including hypotheses giving a pivotal role to sea ice, have revitalized the low-order models used to simulate the time evolution of glacial cycles through the Pleistocene, a prohibitive length of time for comprehensive general circulation models (GCMs). In a recent breakthrough, however, GCMs have succeeded in simulating the onset of glaciations. This occurs at times (most recently, 115 kyr B.P.) when high northern latitudes are cold enough to maintain a snow cover and tropical latitudes are warm, enhancing the moisture source. More generally, the improvement in models has allowed simulations of key periods such as the Last Glacial Maximum and the mid-Holocene that compare more favorably and in more detail with paleoproxy data. These models now simulate ENSO cycles, and some of them have been shown to reproduce the reduction of ENSO activity observed in the early to middle Holocene. Modeling studies have demonstrated that the reduction is a response to the altered orbital configuration at that time. An urgent challenge for paleoclimate modeling is to explain and to simulate the abrupt changes observed during glacial epochs (i.e., Dansgaard–Oescher cycles, Heinrich events, and the Younger Dryas). Efforts have begun to simulate the last millennium. Over this time the forcing due to orbital variations is less important than the radiance changes due to volcanic eruptions and variations in solar output. Simulations of these natural variations test the models relied on for future climate change projections. They provide better estimates of the internal and naturally forced variability at centennial time scales, elucidating how unusual the recent global temperature trends are.

Oceanic oxygen-18 at the present day and LGM: equilibrium simulations with a coupled climate model of intermediate complexity

Abstract An Earth system model of intermediate complexity, CLIMBER-2, is used to simulate the oxygen-18 content of the water masses (H182O) in the oceans. Firstly, we forced CLIMBER-2 with the fluxes from the atmospheric general circulation model ECHAM. Simulated oceanic 18O fields for the present day are in good agreement with data. Secondly, a water isotope module was developed to transport δ18O in the atmosphere on a large scale and compute the 18O fluxes to the ocean at the atmosphere–ocean interface using only variables already computed by CLIMBER-2. For the present day, we successfully represent oxygen-18 distribution in the Atlantic, Indian and Pacific oceans, and close agreement is also found when we compare modelled and observed δ18Ow:salinity relationships. During the Last Glacial Maximum (LGM), we find that the major differences in the 18O oceanic fields (apart from the global oceanic enrichment due to ice-sheet build-up) are due to surface condition changes (surface temperature, shift in bottom water formation zones) and that no drastic changes occurred in the δ18Ow:salinity spatial relationship. In addition, we compute a calcite δ18Oc field for the Atlantic and compare it to the available data to assess the variation between the LGM and the present day.

A simulation of the Atlantic meridional circulation during Heinrich event 4 using reconstructed sea surface temperatures and salinities

With a simplified two-dimensionnal Atlantic Ocean model, we investigate the response of the deep ocean circulation to reconstructed sea surface salinity and temperature used as surface boundary conditions at two time slices: just before and during Heinrich event 4. In contrast to previous studies, we do not make any assumption about freshwater input. Our model results suggest that the recorded estimations of surface hydrological changes during an Heinrich event are able to induce a drastic slowdown, or even a collapse, of the overall Atlantic thermohaline circulation. This is, to some extent, consistent with records of the deep sea ventilation at these times.

Present and Last Glacial Maximum climates as states of maximum entropy production

The Earth, like other planets with a relatively thick atmosphere, is not locally in radiative equilibrium and the transport of energy by the geophysical fluids (atmosphere and ocean) plays a fundamental role in determining its climate. Using simple energy-balance models, it was suggested a few decades ago that the meridional energy fluxes might follow a thermodynamic Maximum Entropy Production (MEP) principle. In the present study, we assess the MEP hypothesis in the framework of a minimal climate model based solely on a robust radiative scheme and the MEP principle, with no extra assumptions. Specifically, we show that by choosing an adequate radiative exchange formulation, the Net Exchange Formulation, a rigorous derivation of all the physical parameters can be performed. The MEP principle is also extended to surface energy fluxes, in addition to meridional energy fluxes. The climate model presented here is extremely fast, needs very little empirical data and does not rely on ad hoc parameterizations. We investigate its range of validity by comparing its performances for pre-industrial climate and Last Glacial Maximum climate with corresponding simulations with the IPSL coupled atmosphere-ocean General Circulation Model IPSL_CM4, finding reasonable agreement. Beyond the practical interest of this result for climate modelling, it supports the idea that, to a certain extent, climate can be characterized by macroscale features with no need to compute the underlying microscale dynamics. Copyright